Update `requests` library to 1.2.3

We're using an out-of-date version of [requests](http://docs.python-requests.org/en/latest/), and we should write code to the latest version and the latest API. This PR also takes references to the [`oauthlib`](https://pypi.python.org/pypi/oauthlib) library that used to be bundled with `requests` and rewrites them to depend on the library as pulled in explicitly, since `requests` 1.0 no longer bundles `oauthlib`. (It's likely that this code could be re-written using [`requests-oauthlib`](https://github.com/requests/requests-oauthlib) to be more concise, but this pull request tries to change as little as possible.)